I also don't see any possible way to make a 'new' model in general to add to the script in question. I'm not the best at scripting but am a quick learner if I have an example to work off of. However, I am a bit confused on what to write.

I assume I need to use MCreator to make a script file which does the whole 'extend' thing so I don't edit the base player model file directly (I remember Minecraft been doing that since 1.2.5 and Starbound also does that when I was modding it). So, I'm trying to make my own replacement (though, very simple and straight to the point). Now, I am using it in 1.12.2 successfully thanks to the help with 'More Player Models' but that mod looks dead and there is no replacement for it. My favorite skin to use in Minecraft is an Argonian skin from the Skyrim Mash-up pack for Bedrock Edition. And now that most of my mods are updating to 1.15.2, I am getting ready to do so myself. But then I forgot about the game, and only got it again on Java Edition on version 1.12.2. Okay, so some background: I actually done modding work before for Minecraft but back in 1.2.5 version of the game where I did figure out how to edit models of existing mobs and even remember using a program to have a full visual of the model so I can easily modify it and do it.

More player models skin editor